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An extension of the basic gray-level grouping (GLG) algorithm, selective GLG (SGLG), which groups the histogram components in different segments of the grayscale using different criteria, is introduced along with two new preprocessing methods to eliminate background noise in noisy low-contrast images. The SGLG and its variations have produced results superior to conventional contrast enhancement techniques and they are fully automatic under most circumstances.

A new automatic method, called gray-level grouping (GLG), for contrast enhancement is described. The basic procedure is to first group the histogram components of a low-contrast image into a proper number of bins and the GLG is automatic in most circumstances and is applicable to a broad variety of images.